1. Where a person has some disease of the gut that prevents adsorbsion (pernicious anemia for example) Even in these cases, using high doses by mouth is often adequate
2 Where a person is so severely deficient that they are in immidiate danger, for example alcoholics who are risking brain damage from thiamine deficiency. They are generaly given a large in jection every day for a few days, then transfered to pills

For the average person wanting to supplement their diet, pills are totaly adequate.

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The desire to take medicines is what seperates man from the lower animals - William Ostler